1. Field of the Invention
The invention refers to a device and a method for collecting successively fed flat objects, in particular sheets, according to the preamble of claim 1 or 13.
2. Description of Prior Art
Collecting means of the type mentioned above are, for example, used in paper handling apparatuses for putting together successively fed sheets to form desired groups of sheets or sheet stacks. Thus, sheets which are for example provided by cutting means and/or sheet feeding means and then advanced subsequently individually, in a shingled or even pre-collected manner are put together to form a sheet stack which can then be transported for further processing to a downstream processing station, such as, for example, folding means, enveloping means or the like.
EP 0 528 493 B1 teaches a generic device for collecting a number of sheets fed one after the other to form stacks. Here, the individual sheets fed one after the other are transported to a stop between an upper and a lower endless transport belt. This includes a stop gate which may be rotated between an upper pivot position where the sheets are slowed at the stop gate and put together to form a stack, and a lower pivot position for releasing the stack formed. With such a collecting device, however, a collected group of sheets must at first be transported completely past the stop gate rotated downwards before it can be rotated upwards again for collecting the next group of sheets. A relatively large gap must be provided between the last sheet of a previous group of sheets and the first sheet of a subsequent group of sheets in order to ensure proper removal of the collected group of sheets. Because the feeders arranged in an upstream transport direction of the collecting device usually allow considerably higher conveying rates than the collecting device, suitable measures must be provided to be able to interrupt or delay the sheet feed to the collecting unit correspondingly. This, however, entails considerable driving and controlling complexity. Additionally, the throughput of a paper handling apparatus and thus of its clock performance is limited by such a collecting device.
EP 0 455 494 B1 discloses means for collecting sheets which comprises two separate gathering units arranged one above the other for improving the throughput. Each of the two gathering units contains upper and lower endless transport belts guided over idler pulleys, between which a collecting station for producing a desired stack of sheets is formed. For collecting the sheets fed one after the other to a gathering unit, opposing bearing rolls are arranged on the respective driving shafts of the, in a transport direction, back idler pulleys, wherein these rolls may be stopped for forming a stack of sheets and be released again to rotate for removing the collected stack of sheets. Between a sheet feeder and the two gathering units arranged one above the other, redirecting means is provided by means of which the sheets fed to the collecting means may either be directed to the upper or the lower gathering unit. Thus, too long a stop of the sheet feeder during normal operation can be avoided. However, in this well-known collecting device, too, a gap must be provided between the last sheet of a previous group and the first sheet of a subsequent group in order for the redirecting means to rotate. Furthermore, the structural complexity and the space requirements are increased by the two-storied design of the gathering unit.